Most simplistically, adhesives are used to bond, whether temporarily or permanently, one or more materials together. Thus, for example, thin films, fibers or small particles that cannot readily be combined, if at all, by other techniques, are readily bonded with adhesives. Typical of such applications include glass wool insulation, fiberglass mat composites, abrasive wheels, sandpaper, emory cloth, and the like.
So too, as a result of using adhesives, stresses are desirably distributed over wider areas, making possible lighter and stronger assemblies than could be achieved with mechanical fastening. For example, airplane wings, tails, and fuselages may be constructed of sandwich panels comprising a honeycomb core bonded to thin faces of aluminum or magnesium, consequently the possibility of fatigue failure is decreased.
Moreover, as a result of adhesive bonding, the strength-to-weight ratios and dimensional stability of anisotropic materials can be improved. As an example, wood, which inherently is non-uniform and water-sensitive, may be converted into a warp-resistant, water-resistant plywood. Nonwoven fabrics having the same properties in all directions are also made by bonding a random web of fiber.
Other advantages of using adhesive bonding in lieu of other methods of joining materials are well known to those skilled in the art. As a result, more and more applications are being found for the utilization of adhesives and, consequently, the use of adhesives is ever increasing.
However, a large portion of the adhesives used in industry today utilize organic solvents and/or non-aqueous diluents to act as viscosity reducers so as to enable these adhesives to be applied to the substrates by various application techniques. Because of increased environmental concern, however, efforts have begun to reduce the pollution resulting from industrial operations, particularly to reducing the amount of organic solvent vapors and non-aqueous diluent vapors entering the atmosphere.
While the primary focus to diminish the emission of such organic solvent vapors and volatile non-aqueous diluent vapors is currently upon the painting and finishing operations, it is just a matter of time before such focus is directed to other operations, such as the application of adhesives.
While there are water-borne adhesive compositions which can be used in lieu of solvent-borne adhesives, such water-borne adhesives are generally limited to particular end uses and applications and are typically less resistant to corrosive environments than are the more conventional solvent-borne adhesive systems.
Moreover, there are adhesive materials which simply do not lend themselves to being used in a water-borne system but which are particularly desirable for specific end use applications and would therefore still require that the medium by which they are applied to substrates be a solvent-borne system. So too, certain classes of adhesives, such as epoxide adhesives, while not requiring a solvent per se, nevertheless require curing agents in order to set these epoxide resins which curing agents also contribute to atmospheric pollution.
Simply reducing the amount of organic solvent, non-aqueous diluent, and/or curing agent may solve the pollution problem, but introduces new problems as to the ability of handling these compositions, applying such compositions to the substrate, such as by spraying, and may even affect the shelf stability, and the curing and/or crosslinking rate or even require higher temperatures to effect an adequate adhesive onto the substrate.
Moreover, water-borne adhesives also suffer from the disadvantages of being subject to freezing, undesirably affecting some of the substrates to which they are applied, such as shrinking fabrics, or wrinkling and curling paper, and may be contaminated by some metals used for storage and application.
While adhesives may be applied to a substrate by various means, such as by brushing, rolling, dipping, and the like, where possible, it is generally more desirable to spray the adhesive composition where the particular adhesive composition, its viscosity and the specific end use permit. Spraying the adhesive composition is particularly desirable when applying an adhesive to substrates such as paper, cardboard, wood, leather, plastic and cloth.
However, by reducing the organic solvent and/or non-aqueous diluent that is present in a sprayable adhesive composition, the viscosity of such a composition generally increases to a point such that it can no longer be sprayed or, if sprayable, the adhesive layer that is applied onto the substrate would not have enough solvent present to allow for sufficient flow-out if a uniform, substantially continuous adhesive coating were desired.
Clearly, at least in the spray application of adhesive compositions, what is needed is an environmentally safe, non-polluting diluent that can be used to thin very highly viscous polymeric adhesive coating compositions to liquid spray application consistency. Such a diluent would allow utilization of the best aspects of organic solvent-borne adhesive applications and performance while reducing the environmental concerns to an acceptable level. Such an adhesive system could meet the requirements of shop- and field-applied liquid spray adhesive coatings as well as factory-applied adhesive coatings and still be in compliance with environmental regulations.
By virtue of the present invention, such a needed diluent has now been found. By the utilization of supercritical fluids, such as supercritical carbon dioxide fluid, as diluents in highly viscous organic solvent-borne and/or highly viscous non-aqueous dispersion adhesive coating compositions, it is now possible to dilute these compositions to application viscosity required for liquid spray techniques.
In addition to the above noted advantage, namely, being able to reduce the amount of environmental pollution caused by the use of volatile solvents, the utilization of supercritical fluids to act as a diluent with adhesive compositions intended for spray application provides yet even further significant advantages.
Specifically, for example, once an adhesive composition is applied to the substrate to be coated, the organic solvents and/or non-aqueous diluents must generally then be removed. Porous substrates, such as paper, permit the liquid solvent and/or diluent to be drawn away from the glue layer. If, however, both adherends are impermeable it is then necessary to evaporate the solvent before mating the two surfaces that are to be bonded, thereby slowing production. Clearly, by the utilization of a supercritical fluid, the amount of such volatile solvent and/or diluent can be significantly reduced thereby reducing, in turn, any problems that may be associated with the removal of such solvent and/or diluent from the coated substrate.
Furthermore, adhesive compositions exist whose properties are such that they entrap solvents and/or diluents and thereby retard subsequent curing. This generally tends to reduce the ultimate cohesive strength of the bonded composite. Here again, by the use of supercritical fluids, which are used to replace a significant portion of such volatile solvents and/or diluents, this problem too is materially reduced.
Yet another example in which the addition of supercritical fluids would be advantageous for reasons other than its impact on environmental pollution is its ability to improve the quality of the spraying application itself. Particularly, it is not uncommon for both cobwebbing and stringing to manifest itself during the spraying of adhesive compositions. However, by virtue of the present invention in which supercritical fluids would be present during the spraying application of such adhesive compositions, such spraying would now be characterized by more explosive atomization caused by the presence of the supercritical fluids thereby significantly reducing, if not entirely eliminating, such undesirable cobwebbing and stringing.
Accordingly, it is clear that the utilization of supercritical fluids with sprayable adhesive coating compositions in accordance with the present invention provides a number of significant advantages, those noted above merely just being exemplary. The utilization of supercritical fluid not only dramatically decreases the amount of organic solvent and/or non-aqueous diluent that needs to be present in an adhesive formulation but, moreover, helps to significantly reduce the spraying and/or adhesive coating formation problems that have been experienced in this art.
In this connection, it is understood that the present invention is particularly suitable for those adhesive coating compositions which have heretofore been applied by spraying techniques. However, the present invention is not limited to such compositions. Indeed, adhesive coating compositions which heretofore have not been able to be sprayed due to their high viscosities may now very well be sprayed by being admixed with supercritical fluids in accordance with the present invention.
The use of supercritical fluids as a transport medium for the manufacture of surface coatings is well known. German patent application 28 53 066 describes the use of a gas in the supercritical state as the fluid medium containing the solid or liquid coating substance in the dissolved form. In particular, the application addresses the coating of porous bodies with a protectant or a reactive or nonreactive decorative finish by immersion of the porous body in the supercritical fluid coupled with a pressure drop to effect the coating. The most significant porous bodies are porous catalysts. However, the applicant characterizes fabrics as porous bodies.
Smith, U.S. Pat. No. 4,582,731, patented Apr. 15, 1986, and U.S. Pat. No. 4,734,451, patented Mar. 29, 1988, describe forming a supercritical solution which includes a supercritical fluid solvent and a dissolved solute of a solid material and spraying the solution to produce a "molecular spray." A "molecular spray" is defined as a spray "of individual molecules (atoms) or very small clusters of the solute." The Smith patents are directed to producing fine films and powders. The films are used as surface coatings.
In Japanese Patent Application No. SHO 57/1982-52890 (Publication No. SHO 58/1983-168674), there is disclosed an adhesive composition for spraying comprising an .alpha.-cyano acrylate type adhesive which is dissolved in liquified carbon dioxide gas such that the adhesive composition is in a liquid state and maintained at room temperature in a pressurized vessel. When sprayed into atmospheric pressure, it assumes a powder, snow-like form having particle diameters in the range of between one and several microns. Only the .alpha.-cyano acrylate adhesive contacts the substrate to be bonded with essentially all of the powder, snow-like dry ice having sublimated. Moisture from the air that has condensed on the snow-like powdered particle surfaces of the dry ice is allowed to remain together with the .alpha.-cyano acrylate and it acts to promote the adhesion effect of the .alpha.-cyano acrylate onto the substrate.
Sprayable adhesive coating formulations are commonly applied to a substrate by passing the adhesive formulation under pressure through an orifice into air in order to form a liquid spray, which impacts the substrate and forms a liquid adhesive coating. In the adhesive coatings industry, three types of orifice sprays are commonly used; namely, air spray, airless spray, and air-assisted airless spray.
Air spray uses compressed air to break up the liquid adhesive coating formulation into droplets and to propel the droplets to the substrate. The most common type of air nozzle mixes the adhesive coating formulation and high-velocity air outside of the nozzle to cause atomization. Auxiliary air streams are used to modify the shape of the spray. The adhesive coating formulation flows through the liquid orifice in the spray nozzle with relatively little pressure drop. Siphon or pressure feed, usually at pressures less than 18 psi, are used, depending upon the viscosity and quantity of adhesive coating formulation to be sprayed.
Airless spray uses a high pressure drop across the orifice to propel the adhesive coating formulation through the orifice at high velocity. Upon exiting the orifice, the high-velocity liquid breaks up into droplets and disperses into the air to form a liquid spray. Sufficient momentum remains after atomization to carry the droplets to the substrate. The spray tip is contoured to modify the shape of the liquid spray, which is usually a round or elliptical cone or a flat fan. Turbulence promoters are sometimes inserted into the spray nozzle to aid atomization. Spray pressures typically range from 700 to 5000 psi. The pressure required increases with fluid viscosity.
Air-assisted airless spray combines features of air spray and airless spray. It uses both compressed air and high pressure drop across the orifice to atomize the coating formulation and to shape the liquid spray, typically under milder conditions than each type of atomization is generated by itself. Generally the compressed air pressure and the air flow rate are lower than for air spray. Generally the liquid pressure drop is lower than for airless spray, but higher than for air spray. Liquid spray pressures typically range from 200 to 800 psi. The pressure required increases with fluid viscosity.
Air spray, airless spray, and air-assisted airless spray can also be used with the liquid adhesive coating composition heated or with the air heated or with both heated. Heating reduces the viscosity of the liquid adhesive coating formulation and aids atomization.
In general, adhesive coating compositions that are intended for spray application may be formulated to help provide optimum spraying characteristics (e.g., minimization of cobwebbing or stringing) as well as optimum adhesion properties after the adhesive coating composition has been sprayed by any of the above means onto a substrate and then dried and/or cured. Some of the defects that may occur on the substrate coated with the adhesive include, but are certainly not limited to, the types noted earlier, such as insufficient solvent evaporation, improper curing, and the like, all of which are well known to those skilled in this art.
Obviously, none of the prior art adhesive coating compositions intended for spray application have been formulated with the intent of having such compositions combined with a supercritical fluid as a diluent and then spraying the resultant admixed liquid mixture through an orifice and onto a substrate to form a liquid coating which is then dried and/or cured.
Indeed, prior to the present invention and the inventions described in the above-noted related applications, it was unknown how a high concentration of highly volatile supercritical fluid, such as supercritical carbon dioxide fluid, would affect formation of a liquid spray containing a solids fraction; a diluent fraction in which said solids fraction is dissolved, suspended or dispersed, and a portion of the supercritical fluid. A spray mixture undergoes a large and rapid drop in pressure as it goes through the orifice. Accordingly, one of ordinary skill in the art could theorize that the supercritical spray mixture would produce a foam like shaving cream instead of a spray, because nucleation to form gas bubbles would be so rapid and intense. Alternatively, one of ordinary skill in the art could also expect that the spray mixture would produce a mist or fog of microdroplets instead of a spray, because atomization would be so intense. Another result that one could theorize is that the spray mixture would produce a spray of bubbles instead of droplets. Furthermore, even if a spray were formed, one of ordinary skill in the art could expect that the sudden and intense cooling that accompanies rapid depressurization and expansion of a supercritical fluid would cause the liquid droplets to freeze solid. For example, it is commonly known that the spray from carbon dioxide fire extinguishers produces solid dry ice particles.
In the event that formation of a liquid spray were achieved, there is no assurance that the spray could be used to produce quality coherent polymeric coatings on a substrate. One of ordinary skill in the art could surmise that the liquid droplets would be so small or have so little momentum that they could not be deposited well onto the substrate. One could also theorize that foaming droplets or supercritical fluid dissolved in the coating would produce a layer of foam on the substrate or a coating full of bubbles when these characteristics were not desired in the coating. The liquid coating droplets that are deposited onto the substrate would have a much higher viscosity than the material that was sprayed, because they would have lost most of the supercritical fluid diluent and they would be at a lower temperature. Furthermore, the coating material would contain less volatile organic solvent than normal. Therefore, it in not unreasonable to expect that higher viscosity would prevent or hinder coalescence of the deposited droplets to form a coherent liquid coating; that it would reduce how much the droplets spread out on the substrate, so that thin adhesive coatings could not be produced; and that it would reduce the surface flow that produces a smooth coating, if such were desired. One can further theorize that moisture would condense onto the droplets and harm the coating, because the spray would be cooled below the dew point.
Surprisingly, however, it has been discovered that liquid sprays can indeed be formed by using supercritical fluids as viscosity reduction diluents and that such sprays can be used to deposit quality coherent adhesive polymeric coatings onto substrates.
Moreover, however, after admixing the highly viscous organic solvent-borne and/or highly viscous non-aqueous dispersions adhesive coating compositions with supercritical fluids as a diluent to help reduce the viscosity, it may still be desirable to reduce the viscosity even further but keep the overall amount of supercritical fluid used substantially the same. Alternatively, it may also be desirable to maintain (or lower) the viscosity of the admixed adhesive coating composition and maintain the overall amount of supercritical fluids used substantially the same, but still want to reduce even further the amount of organic solvent in the admixed coating composition.
More specifically, there may be adhesive coating compositions whose initial viscosity is so high that the amount of supercritical fluids that can be admixed with such compositions, without undesirably causing a two phase separation, is insufficient to reduce the viscosity to the point where such composition can properly be sprayed.
Alternatively, since it is known that high molecular weight polymers generally provide adhesive coatings having better adhesive bonding characteristics as well as better solvent resistance, it may be desirable to use such a high molecular weight polymer in an adhesive coating composition in lieu of a similar but lower molecular weight polymer that may be used in order to provide a manageable viscosity suitable for spraying. However, the use of such a high molecular weight polymer introduces an increase in the overall viscosity of the adhesive composition. This increase in viscosity may be such that the amount of supercritical fluids now needed to reduce the viscosity of the composition to a point suitable for spray application may not be obtainable without breaking up the composition into two phases.
Still further, for a given highly viscous adhesive coating composition containing a particular amount of polymeric component and an organic or non-aqueous solvent, respectively, it may be desirable to reduce the amount of such volatile solvents even further. Of course, such a reduction in solvent would inherently result in a corresponding increase in the overall viscosity of the coating composition. Here again, the increase in viscosity may be such that the amount of supercritical fluids needed to now reduce the viscosity of the composition to a point suitable for spray application may not be obtainable.
Clearly, a need also exists to be able to accomplish all of the above objectives as well. Preferably, these objectives should be able to be carried out without the necessity of adding supercritical fluid in an amount which is greater than that originally needed, such that the expected diluent effect of the supercritical fluids can be expected to remain substantially about the same. Of course, if desired, more than the original amount of supercritical fluid may be used, if such amount does not cause the excessive breakup of the composition into two phases.
Accordingly, the present invention provides a means by which the above noted goals may be achieved and, more particularly, provides precursor adhesive coating compositions in which those goals have been manifested.
Still further, a need also exists to provide precursor adhesive coating compositions which in addition to achieving the above objectives are also formulated to:
(a) be particularly compatible for subsequent admixture with a supercritical fluid diluent;
(b) be particularly suitable, once admixed with the supercritical fluid, to help minimize any of the phenomena that may occur which are peculiarly associated with the utilization of such supercritical fluid, which phenomena may interfere with proper atomization of the admixed liquid mixture and/or proper diffusion of the supercritical fluid once atomized; and
(c) provide desirable adhesive coating characteristics such that once sprayed onto a substrate, it will (i) help provide the necessary coalescence of the deposited droplets to form a coherent liquid coating film or (ii) help provide the necessary properties for obtaining a droplet pattern on a substrate, while still helping to minimize any of the other defects noted above and at the same time, still allow for the release of any residual supercritical fluid that may be present after the adhesive coating has been applied to the substrate.
As used herein, a "liquid adhesive coating" is meant to include a substantially uniform, continuous film formed on the substrate that has been sprayed or, alternatively, a discontinuous, random, liquid droplet pattern that is applied to the substrate.
Accordingly, the precursor adhesive coating compositions of the present invention not only fulfill the goals of (1) having an even lower viscosity and/or (2) having even less organic solvent, but they are also particularly suitable for subsequent admixture with at least one supercritical fluid which admixture is then sprayed through an orifice, such as airless spray or air-assisted airless spray methods, to apply an admixed adhesive coating composition onto a substrate. Advantageously, the coatings have desirable bonding characteristics and a desirable predetermined coating pattern, i.e., whether continuous or discontinuous.
It has unexpectedly been found that water may actually be added to an organic solvent-borne adhesive coating composition such that when admixed with supercritical fluids, the water acts as an additional viscosity reduction diluent providing a composition having an even lower viscosity. Most importantly, however, the amount of supercritical fluids that are miscible with this water-containing coating composition remains at least substantially the same as in the composition in which no water is present.
This discovery is quite surprising in that it has been found that materials such as liquid carbon dioxide or supercritical carbon dioxide are only sparingly miscible with water or water-borne polymer mixtures. Yet, when in the presence of at least one organic coupling solvent, quite surprisingly, a relatively large amount of water may be added to the organic solvent-borne coating composition under supercritical conditions while still retaining the supercritical fluid miscibility characteristics of the original composition. In general, up to about 30 percent by weight of water, based on the total weight of solvent/diluent present in the composition, may be added with substantially no reduction in the amount of supercritical fluid contained in the composition.
Accordingly, in the illustration noted earlier in which not enough supercritical fluid could be added to a viscous adhesive coating composition so as to reduce its viscosity to a point suitable for spraying, this problem can now be solved by simply adding enough water to the composition (up to about 30 percent by weight of the total solvent/diluent present), so as to reduce the initial viscosity of the composition, while still keeping the amount of supercritical fluid that is capable of being admixed with the composition the same. In other words, the addition of the water to the composition serves to act as a further diluent to reduce the viscosity of the composition but does not substantially reduce the miscibility of the now water-containing composition with the supercritical fluids. Most importantly, such a viscosity reduction is achieved without adding organic solvent over and above that which was originally present. While a coupling solvent is desirably added to the composition in conjunction with the water addition, as will be more fully discussed hereinbelow, such coupling solvent may be used to replace some or all of the organic solvent present in the original composition such that the total amount of organic solvent in the water-containing composition is less than or equal to the amount contained in the original composition. With such a viscosity reduction in the new water-containing composition, the amount of supercritical fluids that can be admixed with this composition is generally enough to reduce the viscosity further to a point suitable for spraying.
Similarly, in the illustration noted above in which it would be desirable to replace a low molecular weight polymer with a similar polymer having a higher molecular weight, but the amount of supercritical fluids that can be added to the new formulation cannot be increased to compensate for the increase in viscosity, that too can now be accomplished by adding water to the system. The water acts as a further diluent, and in conjunction with the supercritical fluids (the total amount of which remains substantially the same in both the original composition and in the composition containing water), the viscosity of the reformulated composition containing the higher molecular weight polymer is now reduced to the point that the amount of supercritical fluids that can be admixed with the composition is now enough to reduce the viscosity to a point at which it can be sprayed.
Most significantly, in contrast to the above two illustrations in which water is typically added to a composition so as to actually increase the overall amount of solvent/diluent that is present, the present invention has also recognized that water may also be used to actually replace some of the organic solvent in the original composition. In this manner, while keeping the overall amount of solvent/diluent in the composition substantially about the same, it is possible to reduce even further the amount of volatile organic or non-aqueous solvent that is present in the coating composition so as to accommodate, if needed, the ever increasingly stringent guidelines that are being imposed.
Of course, the present invention recognizes that it is not necessary to start with one adhesive composition formulation and then modify it by the addition of water. The present invention clearly encompasses within its scope the formulation of an initial adhesive coating composition which is formulated with water in accordance with the present invention.